The invention relates to a high molecular weight, medium density polyethylene (HMW MDPE). More particularly, the invention relates to a HMW MDPE that is superior to existing high density polyethylene (HDPE), low density polyethylene (LDPE) and linear low density polyethylene (LLDPE). The invention also relates to a multiple zone ethylene polymerization process.
Polyethylene is divided into high density (HDPE, density 0.941 g/cc or greater), medium density (MDPE, density from 0.926 to 0.940 g/cc), low density (LDPE, density from 0.910 to 0.925 g/cc) and linear low density polyethylene (LLDPE, density from 0.910 to 0.925 g/cc). (See ASTM D4976-98: Standard Specification for Polyethylene Plastic Molding and Extrusion Materials.) One of the main uses of polyethylene (HDPE, LLDPE, and LDPE) is in film applications, such as grocery sacks, institutional and consumer can liners, merchandise bags, multi-wall bag liners, produce bags, deli wrap, and shrink wrap. The key physical parameters of polyethylene film include tear strength, impact strength, tensile strength, stiffness and clarity. Tear strength is measured in machine direction (MD) and transverse direction (TD). Total tear strength (the product of MD tear and TD tear) is an indicator of overall tear properties. Critical processing properties on the film line include the output, bubble stability, gauge control (variability in film thickness), extruder pressure and temperature.
HDPE resins are used in a variety of high strength film applications such as grocery sacks, institutional and consumer can liners, and merchandise bags. Bags made from HDPE resins exhibit excellent tensile strength and stiffness compared to LLDPE and LDPE due to the higher density. Film grade HDPE used for high strength applications usually has a high molecular weight and a broad molecular weight distribution. In general, increasing the molecular weight of HDPE gives better physical properties of the film (tear properties and impact strength) and allows the film producers to reach thinner gauges. However, increasing the molecular weight results in higher extruder temperatures and pressures on a film line. Therefore, the molecular weight distribution of HMW HDPE film resins generally are broad to improve the processability on a film line and to lower the extruder pressures and temperatures. However, consumers often complain of the stiffness and noise associated with handling HDPE bags. This deficiency is related to the higher density of HDPE bags.
LLDPE and LDPE resins are also used in film applications because of outstanding tear properties due to lower density. However, LDPE and LLDPE bags suffer from low tensile properties. Therefore, LLDPE cannot be used in applications such as grocery sacks. Furthermore, LLDPE resins generally cannot be drawn down to thinner film gauges like the HMW HDPE resins.
HDPE is produced with two general classes of catalysts: the chromium oxide catalysts (see, e.g., U.S. Pat. No. 3,974,101) and Ziegler catalysts (Eur. Pat. No. 211,624). The chromium oxide catalysts produce HDPE having a broad molecular weight distribution (MWD) while the Ziegler catalysts produce narrow MWD HDPE.
LLDPE is produced by Ziegler catalysts or newly developed single-site catalysts. Single-site catalysts can be divided into metallocene and non-metallocene. Metallocene single-site catalysts are transition metal compounds that contain cyclopentadienyl (Cp) or Cp derivative ligands (see U.S. Pat. No. 4,542,199). Non-metallocene single-site catalysts contain ligands other than Cp but have the same catalytic characteristics as metallocenes. The non-metallocene single-site catalysts may contain heteroatomic ligands, e.g., boraaryl (see U.S. Pat. No. 6,034,027), pyrrolyl (see U.S. Pat. No. 5,539,124), azaborolinyl (see U.S. Pat. No. 5,756,611) or quinolinyl (see U.S. Pat. No. 5,637,660). Single-site catalysts give LLDPE with narrower molecular distribution and more homogeneous comonomer distribution.
Multiple zone ethylene polymerization processes are also known. For example, U.S. Pat. No. 5,236,998 teaches a parallel multiple reactor process for producing a polyethylene using a Ziegler-Natta catalyst. Moreover, U.S. Pat. No. 4,357,448 teaches a two-step polymerization process to produce HDPE. The HDPE resins prepared by these patents have broad molecular weight distributions and improved processability. However, these resins are of densities within the range of typical HDPE.
Co-pending U.S. appl. Ser. No. 09/302,059 now U.S. Pat. No. 6,127,484, teaches a multiple reaction zone process that uses a single-site catalyst in a first reaction zone and a Ziegler catalyst in a later reaction zone. In the zone where a single-site catalyst is used, a low-density polyethylene is produced, while in the zone where a Ziegler catalyst is used, a high-density polyethylene is produced. Thus, the polyethylene resin produced has a relatively low density. The process, however, is complicated by using both single-site and Ziegler catalysts. Moreover, single-site catalysts are usually more expensive.
The invention is a high molecular weight, medium density polyethylene (HMW MDPE). The polyethylene comprises from about 85 to about 98 wt % of recurring units of ethylene and about 2 to about 15 wt % of a C3-C10 xcex1-olefin. It has a density from about 0.92 to about 0.944 g/cc, a melt index MI2 from about 0.01 to about 0.5 dg/min, and a melt flow ratio MFR from about 50 to about 300. It has a multimodal molecular weight distribution comprising a high molecular weight component and a low molecular weight component. The low molecular weight component is from about 35 to about 65 wt % of the polyethylene. The low molecular weight component has an MI2 from about 50 to about 600 dg/min and a density from about 0.94 to about 0.97 g/cc. This unique composition gives the HMW MDPE performance that is superior to existing high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and low density polyethylene (LDPE).
The invention includes a multiple zone ethylene polymerization process. The process uses a Ziegler catalyst and applies multiple reaction zones. The process comprises polymerizing an olefin mixture in a first reaction zone to produce a first polymer; removing some volatile materials, such as hydrogen, from the first polymer; and then continuing the polymerization in a second reaction zone to produce a second polymer. The first reaction zone uses a higher concentration of hydrogen than the second reaction zone. Thus, the first polymer has a lower molecular weight than the second polymer. More than two reaction zones can be used if desirable. The process of the invention further includes compounding the second polymer in the presence of oxygen.